Adhesives have been used for a variety of marking, holding, protecting, sealing and masking purposes. Adhesive tapes generally comprise a backing, or substrate, and an adhesive. One type of adhesive, a pressure sensitive adhesive (PSA) is particularly desirable for many applications.
PSAs are well known to one of ordinary skill in the art to possess certain properties at room temperature including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Materials that have been found to function well as PSAs are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear strength. The most commonly used polymers for preparation of PSAs are natural rubber, synthetic rubbers (e.g., styrene/butadiene copolymers (SBR) and styrene/isoprene/styrene (SIS) block copolymers), and various (meth)acrylate (e.g., acrylate and methacrylate) copolymers. With the exception of several (meth)acrylates, which are inherently tacky, these polymers are typically blended with appropriate tackifying resins to render them pressure sensitive.
Much less common classes of PSAs are those that are polyurethane-based or polyurea-based. Some examples of these types of PSAs are those based on polyurethane-based polymers, such as the polyether/polyurethane polymers described by Allport and Mohajer in Block Copolymers, D. C. Allport and W. H. Janes Ed., (1973) pp. 443-92. Also see U.S. Pat. No. 3,718,712 (Tushaus) and U.S. Pat. No. 3,767,040 (Tushaus). It has been difficult to obtain balanced viscoelastic properties when using polyurethane-based polymers, however, which may explain their infrequent use when preparing PSAs. For example, U.S. Pat. No. 5,591,820 (Kydonieus et al.) indicates that existing polyurethane-based adhesives function either as weak elastics or high viscosity liquids. The former, weak elastics, tend to fail gradually by peeling away from surfaces to which they have been applied. The latter, high viscosity liquids, typically leave a residue upon removal from a surface and their cohesive strength is too low to withstand stresses applied in many applications. A series of polyurethane-based PSA dispersions have been described in U.S. Pat. No. 6,518,359 (Clemens et al.), and U.S. Pat. No. 6,642,304 (Hansen et al.). These polyurethane-based PSA dispersions are prepared from polyols of greater than 2,000 g/mol molecular weight.
Polyurethane-based polymers are typically prepared by reacting an isocyanate-functional material with a hydroxy-functional material. Some examples of polyurethane-based polymers used for formulating PSAs include those described in U.S. Pat. No. 3,437,622 (Dahl). Polyurethane-based polymers are not always desirable, however, because they typically require either a catalyst or external heat source to form the urethane linkages. For example, see U.S. Pat. No. 5,591,820 (Kydonieus et al.).
Furthermore, many polyurethane-based polymers must be crosslinked to have adequate cohesive strength as PSAs. There are two general methods used to crosslink polyurethane-based polymers. One method is chemical crosslinking through the formation of covalent bonds. However, the degree of chemical crosslinking must be carefully controlled so that the moduli of the resulting material is not increased to the extent that peel adhesion and tack are adversely affected. Furthermore, premature gelation of the adhesive and limited pot life of the PSA may also be problematic when using chemical crosslinking to bolster the cohesive strength of a PSA.
A common chemical crosslinking method described in the literature is the use of multivalent components to achieve a crosslinked network in the adhesive composition. For example, the crosslinked network may be formed by incorporating urethane or urea linkages between polyurethane polymer chains. Urea linkages are typically incorporated into the material by, for example, using a polyamine. For example, see JP-07-102,233 (Sekisui Chemical) and JP 62-297,375 (Kao Corp.). Some of the resulting materials purportedly have PSA properties, either in a partially cured state or in the final composition.
For example, U.S. Pat. No. 4,803,257 (Goel) describes a polyurethane adhesive (i.e., having structural or semi-structural properties) comprising a mixture of a polyisocyanate blocked with a phenolic agent and a polyamine curing agent. The composition may optionally include a polyepoxide. In the partially cured state, this composition is said to exhibit properties similar to those of PSAs. The compositions cure at room temperature to reach the full strength of a structural adhesive.
Also, U.S. Pat. Nos. 3,437,622 and 3,761,307 (Dahl) describe preparation of polyurethane polymers suitable for making PSAs, and which can be crosslinked with certain amines. Suitable amines are taught to be aromatic diamines or polyamines with the amino groups sterically or otherwise hindered by negative groups (Cl, Br, I, OH, etc). These negative groups decrease the reactivity of neighboring amino groups. When crosslinked, it is required that the amino groups be unreactive enough so that polyols and isocyanates can react to form polyurethane polymers before the isocyanates extensively react with the amino groups.
A second method of crosslinking polyurethane-based polymers is physical crosslinking. Physical crosslinking of such polymers typically involves incorporation of urea segments in the polyurethane-based polymeric chain using, for example, an amine chain extender. However, polyurea-based polymers are even less likely candidates for formulation into PSAs because polyureas have even higher moduli than the corresponding polyurethanes due to the chemical nature of the urea groups in polyureas. Accordingly, polyureas tend to be more elastic and adhesives prepared therefrom may not have adequate peel adhesion and tack, properties that may be desired for certain applications. For example, see Chen, Z. et al., “The Study of Polyurethanes and Polyureas by Transmission Spectra of Fourier Transform Infrared Spectroscopy,” Gaofenzi Cailiao Kexue Yu Gongcheng 1993, 9(3), pp. 58-62 and Chen, Z, “Study About Effect of Urea and Urethane Linkages on Phase Separation of Segmented Polyurethanes and Polyureas,” Gaofenzi Cailiao Kexue Yu Gongcheng 1990, 6(5), pp. 66-71. The higher moduli of polyureas present a problem when trying to formulate the polyureas into PSAs, particularly PSAs having adequate peel adhesion for many applications. Perhaps because of this apparent difficulty, there are very few descriptions of PSAs that are polyurea-based. Accordingly, polyurea-based polymers, particularly silicone polyurea-based polymers are typically used for release materials, such as those described in U.S. Pat. No. 5,866,222 (Seth et al.).
Another drawback of using polyurethane-based polymers for the formulation of PSAs is the difficulty often experienced in finding suitable tackifiers for the polymers. For example, U.S. Pat. No. 3,767,040 (Tushaus) teaches the use of certain unique tackifiers synthesized from cyclic terpene alcohols and aromatic isocyanates to provide polyurethane-based PSAs. Tushaus teaches, however, that the tackifiers therein were not found to be effective with natural rubber or styrene-butadiene rubbers or polyurethane polymers other than those specifically described therein to provide PSAs.
Polyurea-based polymers provide an alternative to polyurethane-based polymers. Polyureas are preparable by reacting an isocyanate-functional material with an amine-functional material. Advantageously, polyurea-based polymers typically do not require a catalyst or an external heat source for their preparation.
Among the few descriptions of polyurea-based PSAs, organosiloxane-polyurea block copolymers useful as PSAs are described by Leir et al. (EP Patent Publication No. 0 250 248 A2). The organosiloxane-polyurea block copolymers described therein are prepared by the condensation polymerization of a difunctional organopolysiloxane amine with a diisocyanate. The reaction may include an optional difunctional amine chain extender. These copolymers are stated to be useful, when tackified, as PSA compositions.
Leir et al., however, require the preparation of unique diaminopolysiloxanes. Leir et al. also teach that an approximately equal weight of the silicone tackifier resin with respect to the organosiloxane-polyurea block copolymer is used to make PSAs therein. The use of such highly tackified compositions may not be desirable, however, because tackifiers typically include a relatively high amount of low molecular weight impurities, which may be problematic in certain applications. Furthermore, certain such tackifiers generally tend to be relatively expensive.
Polyurea-based pressure sensitive adhesives and heat activated adhesives are described in U.S. Pat. No. 6,824,820 (Kinning et al.). These polyurea-based polymers are silicone free and require less tackifying agent than the corresponding siloxane-based polyurea PSAs.
New and modified adhesive chemistries are continuing to be developed in order to expand the useful properties of the PSAs. An example of a desirable property for PSAs is self wetting. Generally PSAs that are self wetting are ones that are very soft and conformable and are able to be applied with very low lamination pressure. Such adhesives exhibit spontaneous wet out to surfaces. Some examples of pressure sensitive adhesives that are self wetting have been described. EP Patent Publication 539,099 A2 (Sherman) describes laminates with a pressure sensitive adhesive that comprises a low modulus, self-wetting elastomer. The adhesive formulation has three components: an oligomer having either reactive vinyl or acrylate end groups or side chains; an acrylic monomer having n-hexyl, iso-decyl, n-decyl, 2-ethylhexyl, butyl, or lauryl side chains; and a UV sensitive photoinitiator. PCT Publication No. WO 2005/044470 describes a pressure sensitive adhesive comprising at least one radiation cured oligomer and/or monomer which is cured in situ on a substrate in the form of a coating. The adhesive comprises a polymer which comprises a plurality of polyether segments. US Patent Publication No. 2011/0111240 (Yuan et al.) describes an adhesive formulation that includes (a) a prepolymerized cross-linkable non-water soluble acrylic based pressure sensitive adhesive, (b) a cross-linking agent selected from the group consisting of metal chelates, silanes, epoxy-functional compounds, aziridine compounds, multifunctional amines, alkyl halide compounds, multifunctional acids, multifunctional mercaptans, multifunctional epoxy compounds, polyols in the presence of a catalyst, and combinations thereof, and (c) a polyol having a weight average molecular weight in the range of 1,000 to 10,000 g/mol.